1. Field of the Invention
The present invention relates to image forming apparatuses such as copying machines and page printers that use a fixing unit.
2. Description of the Related Art
A conventional electrophotographic printer employs an LED print head or a laser print head for forming an electrostatic latent image. This type of electrophotographic printer includes various units that perform feeding of a recording medium, transporting of the recording medium, forming of images, fixing of the images, and discharging of the fixed recording medium. The respective units operate at speeds according to the type of medium.
Miniaturization and high printing speed are among recent trends in printer technology. One way of increasing the printing speed without additional components is to provide a sensor for detecting the trailing end of the recording medium immediately after the recording medium has passed a fixing unit. The medium is then transported at a higher speed after the recording medium has passed the fixing unit than when the recording medium is passing through the fixing unit, thereby increasing printing speed.
Another important requirement of recent printers is reduction of noise generated during printing. FIG. 31 illustrates the relation between printing speed and noise generated during printing. Noise increases as printing speed increases. This is a problem with conventional printers.
Once the medium has passed the fixing unit, the heat of a fixing roller is no longer lost to the medium and therefore the temperature of the fixing roller begins to rise. Thus, the electric power supplied to a heater element is shut off and the fixing roller is left to rotate in contact with a pressure roller for an amount of time so that the heat in the fixing roller is transferred to the pressure roller. As a result, the overshoot of the temperature of the fixing roller is minimized. From points of view of miniaturization and cost saving, the fixing roller and a medium discharging section are mostly driven by the same motor. In addition, fixing is performed at a high temperature for increasing the printing speed. This implies that the motor for the medium feeding section and image forming sections may be stopped after the medium has been discharged, but the motor for the fixing roller and the pressure roller should continue to rotate for a predetermined amount of time for preventing the fixing unit from overshooting a predetermined temperature. Extended rotation of the fixing roller and pressure roller still continues to generate annoying noise after the medium has been discharged.